In many medical procedures a target tissue is identified by medical imaging (e.g. computerized tomography or fluoroscopy). However, subsequent medical procedures (e.g. biopsy or excision) may be performed after the imaging procedure has been concluded. In some cases the target tissue is similar to surrounding non target tissue. In the case of a needle biopsy, an operative portion of the biopsy tool is hidden from medical personnel within the patient.
A particular type of guided procedure is radiation therapy. In conventional radiation therapy, ionizing radiation applied as a beam from a radiation source outside the body is used to kill a target tissue (e.g. tumor) in a particular region within the body. In regions of the body where the tissue moves relative to external landmarks it is difficult to provide accurate positional information in order to correctly aim the beam. As a result a larger region than the actual target is often irradiated to ensure that the region to be treated is actually subject to therapeutically cytotoxic doses of radiation. Collateral tissue damage often results. Efforts to reduce collateral tissue damage may result in under-treatment of the intended target.